The amount of heat produced by the sun upon the Earth’s surface, is
greatest near the Equator, and diminishes gradually towards the Poles.
Three general causes, each referable to the spherical form of the Earth,
combine to produce the gradual diminution of temperature from the
Equator to the Poles.
1. The angle at which the Sun’s rays strike the surface. In the
Equatorial regions they are perpendicular to the surface of the sphere,
and there produce their maximum effect; but, on account of the curved
outline of the globe, they fall more and more obliquely with increasing
latitude, and the intensity of action diminishes proportionately. At the
Poles their effect is practically nothing.
2. The area on which a given amount of heating power is expended, is
least at the Equator, consequently the resulting heat is greatest. The
area covered increases, and the effect diminishes, with the increasing
obliquity of the Sun’s rays in higher latitudes, which, as we have seen
above, results from the spherical form of the Earth.
3. The absorption of heat by the atmosphere, as the Sun’s rays pass
through it, is least where they fall perpendicularly,--that is, in the
Equatorial regions,--and increases, with their increasing obliquity,
towards the Poles.